1. Field of the Invention
The present invention relates to an optical recording method and an optical recording apparatus for recording binary two-dimensional digital data page in an optical recording medium as a hologram.
2. Description of the Related Art
A holographic memory is in the limelight as a computer file memory of the next generation which has large capacity derived from three-dimensional recording areas and high speed derived from a two-dimensional page recording/reconstructing method. The holographic memory allows a plurality of data pages to be recorded within an identical volume, and entire data to be read for each page all at once. Instead of producing an analog image, by converting binary values 0 and 1 of digital data into a digital image such that they are made to correspond to "bright" and "dark", respectively, and recording and reconstructing the digital image as a hologram, the digital data can be recorded and reconstructed. In recent years, there have been proposed a detailed optical system of the digital holographic memory system, evaluation of an S/N ratio and a bit error rate based on a volume multiple recording method, and two-dimensional coding, and headway is being made in research from a more optical point of view such as influence of aberration of an optical system.
FIG. 8 shows a shift multiplexing system as one example of a volume multiple recording system described in the literature "D. Psaltis, M. Levene, A. Pu, G. Barbastathis and K. Curtis; OPTICS LETTERS Vol. 20, No. 7 (1995), p782".
The shift multiplexing system uses signal light 31 and a spherical wave as reference light 32 irradiated onto a hologram recording medium 35, and overwrites a plurality of holograms in the same area by rotating the hologram recording medium 35 of disk shape. For example, letting a beam diameter be 1.5 mm, shifting the disk 35 as short as tens of micrometers enables another hologram to be recorded in almost the same area without causing crosstalk. This uses the nature that shifting the disk 35 is equivalent to changing angles of the reference light 32, which is a sphere wave.
A shift distance of the spherical reference wave shift multiplexing, that is, a distance .delta. with which mutual holograms can be separated without significant crosstalk, is represented by the following expression as described in the above described literature. EQU .delta.spherical=.delta.Bragg+.delta.NA .apprxeq.(.lambda.zo/Ltan.theta.s)+.lambda./2(NA) (1)
where .lambda. is the wavelength of signal light, zo is the distance between an objective lens for forming a spherical reference wave and a recording medium, L is the film thickness of the recording medium, .theta.s is a crossing angle of the signal light and the spherical reference wave, and NA is the numerical aperture of the above described objective lens.
It can be found from the expression (1) that the greater the film thickness L of a recording medium, the smaller the shift quantify .delta. and the greater multiplicity, contributing to an increase of recording capacity. Furthermore, the recording capacity could be increased more effectively in the shift multiplexing system by making a recording area minute. Multiple recording in a minute area makes it possible to achieve volume multiplexing of higher density.
To achieve the above object, the holographic memory system subjects signal light to Fourier transform by a lens before making irradiation to a recording medium. By this process, when an image of the signal light has a fine pitch (high spatial frequency), the signal light is subjected to large Fraunhofer diffraction on the recording medium and the spread .zeta. of the diffracted image is represented by the following expression. EQU .zeta.=k.lambda.f.omega.x (2)
where k is a proportionality constant, .lambda. is the wave length of signal light, f is the focal length of a lens for Fourier transform, and .omega.x is the spatial frequency of signal light.
Accordingly, if a lens whose focal length f is small is used as a lens for Fourier transform, a recording area can be made minute. This is described in Chapter 7 of "Holography" (The Institute of Electronics and Communication Engineers of Japan), for example.
Furthermore, a recording area can also be made minute by disposing an aperture forwardly of a storage medium. As described in, e.g., Japanese Published Unexamined Patent Application No. Sho 55-41480, by disposing forwardly of a recording medium an aperture board 39 having a circular aperture 38 whose light transmittance increases gradually toward the center thereof as shown in FIG. 9, meaningless spread of signal light and reference light is limited to make a recording area minute.
However, mere limitation of meaningless spread of signal light and reference light by a circular aperture as in the above described Japanese Published Unexamined Patent Application No. Sho 55-41480 is not sufficient to make a recording area minute to a desired level. Particularly, when binary two-dimensional digital data is recorded in an optical recording medium as holograms, in order to increase a recording capacity, a storage area must be made minute correspondingly to the two-dimensional digital data by limiting the spread of signal light and reference light without causing loss of the data and a read error during reconstructing.